292 MOEPHOLOGY AND EVOLUTION
The anterior part of the insula is exposed as in the other Apes. Its bounding superior limiting (S.L.S.) and fronto-orMtal (F-O.S.) sulci may approach one another very closely, but they do not meet. The fronto-orbital sulcus joins the stem of the Sylvian fissure mesially; and it ascends on the hemisphere, lying almost parallel to the arcuate sulcus behind and the orbital sulcus (O.S.) in front* The latter is long, linear and ascending, and forms a triradius with a long sulcus which runs forwards from it; above the long branch there is a smaller sulcus diverging from the parent stem.
The Sylvian fissure (L.S.J begins in a vallecula on the inferior surface of the hemisphere and runs laterally to gain the outer surface of the hemisphere, whereon it ascends : it terminates in a point or in a bifurcation. Its extremity is encircled by the hook-like posterior end of the parallel sulcus, which sometimes joins the simian sulcus.
The postcentral sulcus (S-P.S.) may be long and complete, or broken up into superior and inferior parts. It, or one of its parts, enters into the formation of the intraparietal complex (I-P.S.)- The horizontal limb of the complex does not always run uninterruptedly into the concurrent transverse occipital and simian sulci (S.S.). The latter, which is not as large as in the Chimpanzee, cuts the mesial border of the hemisphere behind the parieto-occipital sulcus (P-O.S.). Compensatory intraparietal sulci are frequently present.
The region behind the simian sulcus is more fissured than in the Chimpanzee and exhibits inferior occipital <<I-O.S.) and lateral occipital (L.O.S.) sulci. The latter is Y-shaped and embraces the calcarine sulcus (Ga.S.)-